There have been known a variety of recording or printing processes to record or print information such as letters or images with dye or ink on an image-receiving sheet for recording, usually on an image-receiving paper for recording. However, whatever printing process may be employed, the image-receiving sheet for use in such printing processes is in general such that it has a single layer or a plurality of layers on a base sheet formed by coating a solution or dispersion of a suitable substance in a solvent thereon to prevent dye or ink from spreading or to fix dye or ink on the base sheet. Consequently, the conventional image-receiving sheets for such recording processes are expensive on the one hand on account of many steps required for the production, and on the other hand, since any of the printing processes has its own properties, it is needed to use a specially prepared image-receiving sheet for recording to obtain high-quality printing according to the printing process employed.
For instance, for electrophotographic image formation, a method is known for forming multi-color images which comprises selectively exposing a photoreceptor through an original image via a color separator capable of separating the original image into predetermined primary colors, thereby forming a latent image on the photoreceptor, followed by developing the latent image into a visible image corresponding to the primary color with transferring the thus developed visible image on an image-receiving sheet one after another to give a multi-color image on the sheet. For example, with successively transferring the developed visible images of three colors of yellow, magenta and cyan, so-called full-color transfer image duplications can be formed on the image-receiving sheet. This process is a multi-color image-forming process using a so-called, dye-transferring full-color printer.
To such full-color duplication, popularly applied is recording by thermal transfer of sublimable dye, for which, for example, employed is a thermal transfer recording process comprising preparing a thermal transfer sheet that has a sublimable dye layer as formed on a suitable support, such as a polyethylene terephthalate film (this sheet is generally referred to as an ink sheet or an ink film in the art, and will be hereinafter referred to as the former, ink sheet), while, on the other hand, separately preparing a thermal transfer image-receiving sheet having on its surface a receiving layer capable of receiving the sublimed dyes, thereafter laying the ink sheet onto the image-receiving sheet in such a manner that the surface of the dye layer of the former faces the surface of the receiving layer of the latter, then heating the ink sheet with a heating means such as a thermal head in accordance with image information to be transferred onto the image-receiving sheet to thereby thermally transfer the dyes from the ink sheet onto the receiving layer of the image-receiving sheet in accordance with the image information.
The conventional thermal transfer image-receiving sheet for use in such a sublimation thermal transfer recording process is generally produced by lamination through wet-coating of a plurality of resin layers on a base sheet, such as paper, synthetic paper, or suitable synthetic resin sheets, for example, in such a manner that a receiving layer made of resins to which the dyes existing on an ink sheet can be diffused or transferred under heat, and a releasing layer made of resins which acts to prevent the thermal fusion between the receiving layer and the ink sheet are laminated on the base sheet in that order.
Concretely, the conventional thermal transfer image-receiving sheet is produced by applying onto a base sheet a solution comprising resins to constitute a receiving layer on the base sheet, then drying the solution to thereby form the intended receiving layer of the resin on the base sheet, thereafter applying thereonto a solution comprising resins to form a releasing layer, and drying the solution to form the intended releasing layer of the resins on the receiving layer of the resins. Therefore, such a plurality of resin layers each having a different function are laminated on the base sheet. If desired, an undercoat layer or an interlayer may be formed between the base sheet and the receiving layer. Accordingly, the process for producing the conventional thermal transfer image-receiving sheet is complicated, and the production costs are high.
Apart from the recording system of the above-mentioned type, a different, thermal transfer full-color printing process has also been proposed, in which a resin layer is previously laminated on an ink sheet, the resin layer is first thermally transferred from the ink sheet onto an image-receiving sheet to form thereon a receiving layer prior to the transference of yellow, magenta, cyan and black dyes thereonto in that order, and thereafter these dyes are thermally transferred onto the thus formed receiving layer on the image-receiving sheet.
However, this process is problematic in that the first transference of the resin layer takes much time, resulting in the prolongation of the time for the intended full-color printing, that the formation of a uniform receiving layer on common paper is not easy, and that the quality of the transfer image to be finally obtained is poor. In addition, it is further problematic in that the lamination of the resin layer (this layer is, as mentioned above, to be the receiving layer on the image-receiving sheet) on the surface of the ink sheet is technically difficult. At any rate, for the recording process by thermal transfer of sublimable dye, a specially prepared image-receiving sheet for use has has hitherto been needed.
On the other hand, a thermally meltable (i.e., capable of melting) ink transfer printing process is also well known, in which ink on an ink sheet is heated and melted, and is then transferred and fixed on a thermal transfer image-receiving sheet. As seen, the image-receiving sheet for use in thermally meltable ink transfer printing process comprises a base sheet and a microporous resin layer thereon to receive the melted ink. Thus, the thermally meltable ink transfer printing process also needs a specially prepared image-receiving sheet.
An ink jet printing process is also known. This printing process uses aqueous ink jet ink so that it also needs a specially prepared image-receiving sheet for use which comprises a base sheet and a colorant-receiving layer to be dyed and a moisture absorbing layer to absorb excess water in the ink. A typical image-receiving sheet for this ink jet printing process has on a base sheet, for example, a moisture absorbing layer formed of water-soluble resins and a colorant-receiving layer formed of, for example, cationic acrylic resins. Meanwhile, an ink jet printing process in which solid ink is used is also known, in which an image-receiving sheet which has a microporous resin layer on a base sheet to receive the ink is used.
Finally, even in a printing system in which a plate, such as a letterpress, is used, high quality, high darkness printing is obtained without ink spreading only when resin-coated and flat surface paper, such as art paper, calender roll paper or offset paper is used to receive printing ink effectively.
As described above, whatever printing process may be employed, it has been necessary to use a specially prepared image-receiving sheet which has on a base sheet a dye- or ink-receiving layer in a single layer or in a plurality of layers according to the printing process employed so that a high quality printing or image is realized. On the contrary, when common paper is used as an image-receiving sheet, a desired high-quality printing or image has not been realized. Thus, so far, any of the printing processes mentioned above produces a high quality printed image when an image-receiving sheet specially prepared so as to be suited to the process employed is used, but this apparently costs a great deal.
The use of such a specially prepared image-receiving sheet involves further problems. Very often the conventional sheet has a very flat surface, or on the contrary it has a very porous surface according to the printing process in use. In particular, since many of the conventional thermal transfer image-receiving sheets have on base sheets dye- or ink-receiving layers and releasing layers formed by wet-coating so that such dye- or ink-receiving layers are excessively flat and glossy. That is, usually the dye- or ink-receiving layers have a surface roughness Ra in the range of 0.2-0.4 and a ten point average roughness in the range of 1.5-2.0 as measured in accordance with JISB 0601-1994. Thus, it is difficult to write on such a flat surface with a common writing instrument such as a pencil, fountain pen or ball-point pen. It is also difficult to obtain a grayed printed image having a feeling of quality.
The conventional thermal transfer image-receiving sheet is generally produced through wet-coating of a plurality of resin layers each having a different function laminated on a base sheet. Accordingly, when common paper is used as the base sheet, it is usually difficult to form receiving layers on both sides of common paper. That is, it is not possible to form thermal transfer images on both sides of common paper.
Moreover, the conventional thermal transfer image-receiving sheet has, in general, a receiving layer only on the front of the base sheet and hence has a different layer structure on the front from that of the back so that it is apt to curl depending upon the ambient humidity or temperature conditions to reduce commercial value. In particular, when paper is used as a base sheet and a receiving layer is formed on the front, the base paper absorbs moisture and swells under a high humidity whereas the receiving layer is low in absorbency since it is formed of resins so that the image-receiving sheet curls and hence is reduced in commercial value. As a further problem, a thermal transfer image-receiving sheet is placed under a high temperature of 200-500.degree. C. momentarily when an image is thermally transferred from an ink sheet. Thus, when the sheet contains moisture, it evaporates very rapidly and the sheet curls remarkably.
As described above, the conventional image-receiving sheets for recording with dye or ink, especially such a sheet in which paper is used as a base sheet, have a plurality of layers such as receiving layers and releasing layers formed by multi-step wet-coating processes on the base sheet, and accordingly they are expensive as well as they have a variety of problems as stated above.
To cope with these problems, there has been proposed a process for the production of an image-receiving sheet for sublimation thermal transfer recording which comprises dry-coating a powdery coating composition which contains a resin component therein on a base sheet, and heating, melting and fixing the powdery coating composition on the base sheet to form a dye- or ink-receiving layer comprised of a continuous resin coating or film, as disclosed in Japanese Patent Application Laid-open No. 8-112974. According to the process, a receiving layer can be easily formed on a base sheet, even if paper is used as a base sheet. Accordingly, the process provides a thermal transfer image-receiving sheet in an inexpensive manner.
However, the image-receiving sheet thus produced has other problems. In particular, since paper is comprised of cellulose fibers and has an uneven or undulating surface, when it is used as a base sheet and a receiving layer formed thereon is thin, the layer follows the uneven or undulating surface. As results, when an ink sheet is attached to the image-receiving sheet under heat to transfer the dye of the ink sheet to the image-receiving sheet, a clear image cannot be obtained on account of lack of uniform contact between the ink sheet and the image-receiving sheet. This tendency is remarkable especially when the surface of a base paper has an unevenness or undulation not less than 10 .mu.m in height.
The invention has been made in order to solve the above-mentioned problems associated with the conventional various printing processes, in particular, image-receiving sheets and their production.
Specifically, it is an object of the invention to provide a simple and inexpensive process for producing an image-receiving sheet having a dye- or ink-receiving layer on a base sheet, preferably on paper, for use in a variety of printing processes to form high quality images thereon, preferably image-receiving sheet for recording by thermal transfer of sublimable dyes or thermally meltable inks, ink jet printing or plate printing. It is also an object of the invention to provide a process for producing such image-receiving sheets for recording by such printing processes.
More specifically, it is an object of the invention to provide a process for producing an image-receiving sheet easily and inexpensively, if necessary, by use of a long-size continuous base sheet, for use in any of printing processes by thermal transfer of sublimable dyes or thermally meltable inks, ink jet printing process or plate printing process to form high quality images, by dry-coating a powdery coating composition by an electrostatic spraying process on a base sheet, and heating, melting and fixing the composition thereon to form a dye- or ink-receiving layer.
A further object of the invention is to provide a thermal transfer image-receiving sheet which comprises a base sheet and a single receiving layer thereon comprised of a powdery coating composition, and yet has a good releasability from an ink sheet, and moreover which is produced by a simple process.
A still further object of the invention is to provide a thermal transfer image-receiving sheet which has a dye- or ink-receiving layer having a predetermined thickness on a base sheet, in particular, a base paper, to compensate or offset the unevenness or undulation of the surface of the base paper, and which accordingly can form a clear image with no defect.
It is also an object of the invention to provide a thermal transfer image-receiving sheet which has a receiving layer of which surface is moderately uneven, that is, matted, so that it forms an image having a feeling of quality and an ordinary writing instrument writes well on the sheet.
It is still an object of the invention to provide a process for producing a two-layer structure thermal transfer image-receiving sheet which has a receiving layer on a base sheet and a releasing layer thereon so that it has good releasabilty from an ink sheet.
In addition to above, a still further object of the invention is to provide a thermal transfer image-receiving sheet which has a receiving layer on the front of a base sheet and a receiving layer or a resin layer which is not receptive to dye or ink on the back of the base sheet so that the sheet can receive images on both sides and/or the sheet is free from curling under influence of ambient humidity or temperature.